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There are two main types of antihistamines, which are described below.

Sedating

The older types of antihistamines are sometimes referred to as sedating antihistamines. This type of antihistamine can affect your brain, causing you to become drowsy. An example of a sedating antihistamine is chlorphenamine maleate (eg Piriton).

Non-sedating

The newer types of antihistamines are sometimes referred to as non-sedating antihistamines. This type of antihistamine has less of an effect on your brain and won’t make you so drowsy. Acrivastine (eg Benadryl) is an example of a non-sedating antihistamine.

Your immune system protects you from harmful substances such as bacteria and viruses. It does this by producing antibodies which help to remove these from your body.

In an allergic reaction, your immune system reacts to a harmless substance such as pollen, because it mistakes it for a harmful one. When this happens, a chemical called histamine is released by your immune system. Histamine is very useful because it helps damaged tissue to heal. However, this process can also cause symptoms such as:

a runny nose

itchy eyes, nose, throat and skin

sneezing

urticaria

Antihistamines work by blocking the effect of histamine in your body. This helps to prevent inflammation and eases allergic reactions.

If you have a severe allergic reaction, your symptoms may include:

difficulty breathing

swollen lips and eyelids

a fast heartbeat

This type of allergic reaction is called anaphylaxis. If you have anaphylaxis, it’s important that you get medical help immediately as it can be very serious. You will be given an injection of adrenaline if you have anaphylaxis. Sometimes an injection of an antihistamine is used in addition to adrenaline.

You can buy some antihistamines from pharmacies without a prescription. Examples include chlorphenamine maleate (eg Piriton), loratadine (eg Clarityn) and cetirizine (eg Zirtek). Some antihistamines are only available on prescription from your GP as they could have side-effects or interactions with other medicines you might be taking.

Depending on what you need antihistamines for; you can take them as tablets, liquids, nasal sprays or a cream. Only use antihistamine creams for a short amount of time as they may cause an allergic reaction. Don’t use antihistamine creams on areas of broken skin, unless your GP has said that you can do so.

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If you’re pregnant, it’s recommended that you don’t take antihistamines. This is also true if you’re breastfeeding because antihistamines can get into your breast milk. If you feel you need to be treated with antihistamines while pregnant or breastfeeding, see your GP for advice.

If you have epilepsy, you must get advice from your GP before taking a sedating antihistamine. You should not take this type of antihistamine if you have severe liver disease.

This section doesn’t include every possible side-effect of antihistamines. Always read the patient leaflet that comes with your medicine and ask your pharmacist for advice.

The sedating forms of antihistamines will make you feel very drowsy and can affect your coordination. Therefore, it’s important that you don’t drive or drink alcohol for 24 hours after taking a sedating antihistamine. Alcohol can increase the sedative effect of these types of antihistamines. You should not drink alcohol if you’re taking these antihistamines.

With the newer, non-sedating antihistamines drowsiness is less of a problem.

Side-effects that are more common with the older, sedating antihistamines include:

Check with your GP or pharmacist before you take any other medicines at the same time as antihistamines.

Tricyclic antidepressants interact with antihistamines and can increase the drowsiness side-effect. The antihistamine mizolastine can also interact with some other medicines and may cause a serious abnormal heart rhythm. This medicine is only available on prescription from your GP, who will give you advice if you need to take it.

Some antifungal (ketoconazole) medicines and antibiotics (erythromycin) can increase the amount of non-sedating antihistamine in your body.

It’s best not to drink alcohol while taking antihistamines of any type as this will increase the drowsiness side-effect.

What should I do if I take too much antihistamine?

Answer

It’s possible to overdose on antihistamines. If you do, it’s important to tell your GP or pharmacist straight away. Go to your nearest accident and emergency department if you feel unwell. Take the medicine packet(s) with you, so your GP, pharmacist or hospital doctor can see exactly what type of antihistamine you have taken.

Explanation

The effects of taking too much antihistamine are similar to the side-effects of this medicine but more severe. If you take too much antihistamine, you might feel very sleepy, dizzy or confused, especially if you have taken one of the older, sedating antihistamines. Alternatively, you may feel wide awake. Other effects of taking too much antihistamine include breathing difficulties, fits and arrhythmia (abnormal heart rhythm).

Many different medicines contain antihistamine, such as those for coughs and colds. You could easily reach the maximum safe dose if you take an antihistamine at the same time as taking one of these medicines. If you have any questions, ask your pharmacist for advice.

What is better for hay fever - antihistamine nasal sprays or tablets?

Answer

The type of antihistamine that you choose to treat your hay fever (allergic rhinitis) will depend on the symptoms you have, such as whether you’re mainly sneezing or if your eyes are itchy.

Explanation

If you have occasional symptoms, taking antihistamine tablets when you need them may be a good first option. They might also help if you have allergic conjunctivitis or for children aged between two and five years who have hay fever. You can also use a nasal spray. This might relieve some of your symptoms quickly as it usually works within 15 minutes.

If your symptoms are more frequent or persistent, try and stay away from what is causing your allergy, for example pollen. If you can’t do this, a steroid nasal spray or drops might help to treat your symptoms.

If your symptoms mainly affect your eyes, your GP may prescribe you eye drops. It’s important to remember that eye drops may cause discomfort in some people.

Talk to your pharmacist or GP if you're unsure which treatment is best for you.

Do antihistamines become less effective if you use them for a long time?

Answer

You’re unlikely to develop a tolerance to antihistamines if you use them to treat allergies, even with daily use. Tolerance means you will need to take higher doses of the medicine for it to have the same effect.

Explanation

Some studies have shown that if you use antihistamines daily to prevent symptoms of hay fever and other allergies, they may have the same effectiveness even after a long period of taking them.

The same isn’t true of the antihistamines that are used to treat insomnia, such as promethazine hydrochloride (eg Phenergan). The sedative effect of antihistamines reduces over time. If you have long-term insomnia, see your GP for advice. This is because all medicines for insomnia should only be used in the short term.

If you’re finding that your symptoms aren’t well controlled by the antihistamine treatment you’re using, speak to your GP or pharmacist. It might be that you’re not taking the medicine in the most effective way. For instance, if you’re using an antihistamine nasal spray, it’s possible that you’re not applying it correctly. Or you may find that you need additional treatment, such as a steroid nasal spray.

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